OFDM (Orthogonal Frequency Division Multiplexing) is one type of multicarrier modulation method and is a method for performing communication by transmitting divided data on multiple subcarriers which are orthogonal to each other. Each of the subcarriers is modulated by QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation) or the like. When the subcarriers are orthogonal to each other, each of the subcarriers has a frequency in which the power of the other subcarriers is set to zero as shown in FIG. 1. Thus, the subcarriers can be densely arranged so that their waveforms are overlapped with each other, allowing reduction in the frequency bands to be used. Furthermore, the data is allocated to each of the subcarriers, and thus a data transfer rate in each of the subcarriers can be lowered compared with the case of a single carrier. Consequently, a time, called a guard interval, for absorbing a delayed wave can be provided for each symbol.
Moreover, in a multipath environment, a fading phenomenon that a reception level of a certain frequency drops may occur. However, even if the data is partially lost, OFDM can reduce influences of the lost data by interleaving and error correction because the data is distributed to the subcarriers in OFDM. OFDM is usually utilized in combination with a multiple access method such as FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access).
Meanwhile, OFDMA (Orthogonal Frequency Division-Multiple Access) is the same as OFDM in dividing a carrier into multiple subcarriers as shown in FIG. 2. OFDMA is a method for achieving multiple access by sharing all the subcarriers among all users, grouping a certain number of subcarriers as a subchannel, and adaptively allocating the subchannel to each of the users at any timing.
Note that Japanese Patent Application Publication No. 2005-229468 discloses a technique of receiving, from a communication partner device, a control signal used to provide a radio frequency band and a transmission frequency, and allowing a radio frequency band and a transmission frequency of an OFDM-modulated radio transmission signal to be changed according to the control signal.
According to the above technique, for example, a control signal specifying a radio frequency band and a transmission frequency for each radio transmission signal is given to each of multiple cameras. Accordingly, each camera is allowed to change a frequency band or a center frequency of the radio transmission signal according to the given control signal. As a result, an output channel of a receiver for allocating a broadband channel and a narrowband channel can be changed as needed. Thus, it is possible to implement a wireless camera system capable of effectively utilizing limited frequency bands.